1. Field of the Invention
The present invention relates to novel oriented composite polyester films having improved adhesiveness towards a variety of final application coatings, to a process for the production thereof, and to the final application coated films fabricated therefrom
By the term "final application coatings" as utilized herein, are intended coatings which impart certain properties to the oriented polyester films which permit
extremely diverse industrial applications thereof, all well known to this art.
2. Description of the Prior Art
It is known to this art that for a number of industrial applications, polyester films cannot be used directly, but only after depositing thereon a coating adapted for each type of application; as examples, there may be mentioned matte coatings intended for manual drawing or for plotting tables; photosensitive coatings for photographic applications; diazo coatings for microfilm applications, printing of plans or reprography; coatings for printing; magnetic coatings for recording various signals (audio, video and computer signals); metal coatings; and coatings which modify the gas permeability of polyester films employed as packaging materials.
It is also known to this art that, by nature, the bonding of the final application coatings onto the surface of polyester films is not easy. In fact, polyester films have a relatively smooth surface, limited chemical reactivity, and a very low sensitivity to solvents. Therefore, it is most often necessary to first apply thereto an "adhesion primer" coating of more or less complex chemical formulae, which enables the application layers to be anchored. In general, this adhesion primer (or anchoring layer) is applied by the individual conducting the conversion. Therefore, it is easy to adapt the nature of the primer to the application ultimately made.
Film manufacturers have themselves sought to modify the surface of films by various means. The use of polymers of diverse nature as adhesion primers has been proposed for this purpose. A particularly advantageous class of polymers employed for this purpose consists of copolyesters having a plurality of recurring hydrophilic groups and especially hydroxysulfonyl groups, or their metal salts (free sulfonic acid groups or sulfonic acid salts will hereinafter be referred to by the term "oxysulfonyl group" for convenience). Adhesion primers of this type may be deposited either by coating the support film with a dispersion or an aqueous solution (cf. French Patents Nos. 1,401,581 and 1,602,002; U.S. Pat. No. 4,476,189 and European Patent No. 78/559), or by coextrusion or rolling (cf. Japanese Patent Applications published under Nos. 50/135,086 and 79/153,883). The deposition of an adhesion primer of the oxysulfonyl group-containing copolyester type by coating involves the use of copolyesters having high contents of oxysulfonyl group-containing recurring units to enable the sulfonated copolyester to be dispersed or dissolved in water. The coating process lacks flexibility because it restricts the range of sulfonated copolyesters which can be employed to those having a high content of oxysulfonyl group-containing recurring units. Moreover, depending on their composition, such sulfonated copolyesters may have low bonding temperatures which make the composite films difficult to handle or to use.
Coextrusion is a particularly advantageous process for applying an adhesion primer coating. However, it is not suitable for depositing a coating of highly modified sulfonated copolyesters onto a crystalline or semicrystalline polyester, either because of the high content of oxysulfonyl group-containing recurring units, or because of the presence of both such units and of recurring units derived from other comonomers employed to impart specific properties to sulfonated copolyesters (for example, isophthalic acid). Thus, it has been observed that sulfonated copolyesters of the ethylene glycol terephthalate/5-sulfoisophthalate type having a high content of 5-sulfoisophthalate units, for example greater than or equal to 10 mol %, do not lend themselves well to the production of composite films by coextrusion. Their use involves a lengthy and expensive drying step because of their hygroscopicity; insufficient drying interferes with the progress of the coextrusion process and results in poor quality films. Additionally, the rheological properties of sulfonated copolyesters are greatly modified by the presence of oxysulfonyl group-containing recurring units and are different from those of the polyester employed to produce the support film. This difference in rheological properties of the two polycondensates complicates the production of a composite film by coextrusion, or even makes it impossible. When the sulfonated copolyester contains recurring units other than sulfonated units, this gives rise to another disadvantage in addition to those mentioned above. Indeed, it has been observed in this case that beyond a certain content of sulfonated recurring units (in general, for contents greater than or equal to 5% relative to the units of the same nature), the composite film has relatively low bonding temperatures, usually less than 100.degree. C. Sulfonated copolyesters having low bonding temperatures are not well adapted for production and/or conversion operations, such that it is usually difficult, if no impossible, to produce a film which can be used on an industrial scale. This is very particularly so when sulfonated copolyesters of the terephthalic acid/oxysulfonyl group-containing dicarboxylic acid/isophthalic acid type derived from ethylene glycol are employed. Moreover, coextrusion entails having available a wide range of sulfonated copolyesters having a content of oxysulfonyl group-containing recurring units adapted for each type of application, which gives rise to storage and production constraints. The need to have available primer layers of varying degrees of roughness also necessitates the availability of sulfonated copolyesters having varied filler contents, which contributes to expanding the range of sulfonated copolyesters which can be employed for the production of adhesion primers by coextrusion.
In summary, the manufacturer of sulfonated copolyester-based adhesion primer coated film is confronted with the following alternatives: using the coating process for the copolyesters having a high content of oxysulfonyl group-containing recurring units or using the coextrusion process for copolyesters having a low content of oxysulfonyl group-containing recurring units in order to avoid the aforementioned disadvantages.
Consequently, serious need exists in the polyester film industry for means permitting a sulfonated copolyester-based adhesion primer to be applied to a crystalline, semicrystalline or crystallizable polyester film irrespective of the content of oxysulfonyl group-containing recurring units in such sulfonated copolyester, and, more particularly, permitting such coating to be applied by coextrusion in the case of copolyesters having a high content of oxysulfonyl group-containing recurring units, especially a content of such units greater than or equal to 5 mol % relative to the bifunctional units of like nature. The industry is also in need of means to limit the number of sulfonated copolyesters which must be available to the manufacturer for the production of coated films which differ in their adhesive properties and/or their topography.